Spectroscopy of VUV luminescence in dual-phase xenon detectors

TL;DR

This work delivers high-resolution, time-resolved spectroscopic measurements of vacuum ultraviolet xenon luminescence in a dual-phase xenon time projection chamber, enabling simultaneous characterization of liquid scintillation (S1) and gas electroluminescence (S2) under thermal equilibrium. Using a monochromator and photon-counting approach, the authors extract both the liquid S1 and the gas S2 spectra and, for the first time in liquid xenon, separate singlet and triplet emission components. The primary scintillation peak is measured near $177.1$ nm with a width of about $11.3$ nm, while the gas electroluminescence peak is near $173.28$ nm with a width of about $10.59$ nm; singlet and triplet liquid emissions are identified at distinct wavelengths ($176.1$ nm and $177.9$ nm, respectively), and room-temperature gas data provide a consistent baseline across pressures. These results, including a small short-wavelength tail in S2, furnish precise spectral inputs for optical models of LXe detectors and significantly improve the understanding of phase- and excitation-mode dependent xenon emission relevant to dark matter and neutrino experiments.$177.1\mathrm{nm}$, $173.28\mathrm{nm}$, $11.3\mathrm{nm}$, $10.59\mathrm{nm}$, $176.1\mathrm{nm}$, $177.9\mathrm{nm}$, $172.27\mathrm{nm}$, $0.6\%$.

Abstract

We present spectroscopic measurements of xenon luminescence in a time projection chamber operated in a dual-phase (liquid-gas) configuration. Thorium-228 $α$ decays excited the liquid, resulting in the formation of singlet and triplet excimers that emit vacuum ultraviolet (VUV) scintillation. Ionisation electrons were drifted to the liquid surface and extracted into the vapour, where they produced VUV electroluminescence. A time-resolved photon-counting technique was used to obtain the scintillation spectrum in the liquid, which exhibited a peak wavelength of $177.1\pm0.1_\mathrm{stat} \pm0.1_\mathrm{sys}\,\textrm{nm}$ and a full-width at half maximum (FWHM) of $11.3\pm0.2_\mathrm{stat} \pm0.0_\mathrm{sys}\,\textrm{nm}$. The data were also used to obtain distinct singlet and triplet emission models, with the singlet emission peaking $1.8\pm0.3_{\mathrm{stat}}\pm 0.3_{\mathrm{sys}}\,\textrm{nm}$ shorter than the triplet. The gas electroluminescence spectrum was obtained simultaneously, while under conditions of thermal equilibrium. It remained consistent across vapour pressures of $1.3$-$2.2\,\textrm{bar}$, with a peak of $173.28\pm0.02_\mathrm{stat} {_{-0.1}^{+0.2}}{}_\mathrm{sys}\,\textrm{nm}$, a FWHM of $10.59\pm0.03_\mathrm{stat} {_{-0.2}^{+0.0}}{}_\mathrm{sys}\,\textrm{nm}$, and a small short-wavelength tail that constitutes $(0.6\pm0.1)$% of the total spectrum. These are the only spectroscopic measurements of liquid scintillation and gas electroluminescence acquired simultaneously to date, and the first such measurements of singlet and triplet emission in the liquid phase. They are important for precisely characterising dual-phase xenon detectors used to search for dark matter particle interactions and other rare events.

Figures (14)

• Figure 1: Potential energy curves for the formation of the ground state xenon dimer [Xe$_2$(X$0_g^+$)] and the first two excited state dimers [Xe$_2^*$(B$0_u^+$), Xe$_2^*$(A$1_u$)]. These are calculations of the potential energy between two xenon atoms in vacuum xiaowei2020nee2000; the influence of other nearby atoms is neglected, therefore these curves should be considered as an approximation in the liquid and gas phases. The energy regimes of fluorescence from the first and second continua are illustrated by arrows. Note that the low-energy region is plotted in logarithmic scale to reveal the shape of the ground state.
• Figure 2: An illustration of the primary components of the optical system and simplified TPC. Xenon luminescence was detected either directly by the internal PMT, or transmitted through two MgF$_2$ viewports and a monochromator to a secondary spectroscopy PMT (not depicted). For calibration purposes, a low-pressure Hg arc lamp was strategically positioned directly opposite the monochromator, ensuring that its emitted light followed the same optical path as the xenon luminescence.
• Figure 3: A set of averaged $\alpha$-decay waveforms in the internal PMT (blue and red pulses) together with a single photon pulse (black) from the spectroscopy PMT, detected during the dataset with liquid and vapour in equilibrium at $2.2$ bar.
• Figure 4: Wavelength-dependent efficiencies of the optical components within the setup. Transmittance calculations for the innermost MgF$_2$ viewport were conducted separately for S1 and S2 photons propagating from liquid and gaseous xenon into MgF$_2$, respectively. These were restricted to wavelengths above $160$ and $150$ nm, corresponding to the spectral region where significant VUV luminescence was observed. The results were combined with the efficiency of the monochromator grating Fujii and the quantum efficiency of the ETL 9406B PMT PMTs, as shown in the bottom panel. The transmittance of the second MgF$_2$ viewport, which separates the outer vessel from the monochromator, exhibits minimal wavelength dependence and is, therefore, not shown for clarity.
• Figure 5: Spectrum acquired from the low-pressure Hg discharge lamp, annotated with the wavelengths of known atomic Hg lines. The inset panels display Voigt profile fits to the atomic line at $184.9$ nm and the doublet of lines at $312.6$ and $313.2$ nm, after having applied corrections for the DAQ deadtime. The $184.9$ nm line serves as a calibration standard within the VUV regime of xenon luminescence, while the doublet illustrates our ability to nearly resolve a spectral feature on the subnanometre scale.